Compact optical fiber coupler

ABSTRACT

The inventive coupling device enables a high interconnection density of single mode optical fiber in active and passive devises used in a fiber optic telecommunication system. The coupling device comprise a micro-lens formed by terminating a single mode optical fibers with an optimized gradient index fiber, thus avoiding a significant increase in fiber diameter. The gradient index is optimized to provide a long working distance to the minimum spot size so that efficient coupling can be achieved in a free space interconnection between either multiple single mode fibers or a single mode fiber to a transmitting or receiving device.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to provisionalapplication having serial number 60/276,730 filed Mar. 16, 2001,entitled Compact Optical Fiber Coupler, which is incorporated herein byreference.

BACKGROUND OF INVENTION

[0002] In an optical communications system optical signals may betransmitted in free space, but are generally transmitted over opticalwaveguides, typically optical fibers. Since optical fiber transmissionoffers tremendous bandwidth and transmission rate advances over thetransmission of electrical signals, conversion to electrical signals areavoided as much as possible by active optical processing such as opticalamplification, switching and routing. It is usually desirable to avoidconversion of the signal to an electrical signals until they reach thetarget destination, where they are converted back to electrical signalsrepresenting digital data, voice or images in various analog formats.

[0003] In order to maximize the capacity of fiber optic communicationsystems many signal are simultaneously transmitted over the same fiberwaveguides in a scheme known as wavelength division multiplexing orWIDM. In WDM each discrete signal may correspond to a differentwavelength of light, known as an optical channel. Various non-linearproperties of optical glass, active and passive components in theoptical system, produce cross talk between the WDM optical signalchannel. This “cross talk” is insignificant if the signal to noise ratiois high and the power levels of all optical channels are comparable.

[0004] The optical devices and interconnections in any route will resultin signal losses, thus the signal power and signal to noise ratio of anyoptical signal can be expected to vary with the routing path. When thecommunication system is a network, optical channels are combined androuted together in common waveguides with signals from different sourcesthe power levels in each optical channels are likely to be different, inwhich case the “cross-talk” from the stronger channels will degrade thesignal to noise ratio in the weaker channels.

[0005] Therefore, low insertion and high isolation is a substantialconsideration in the design and operation of all optical communicationsystem components. While very low losses can be obtained by fusionsplicing optical fibers of similar composition many passive and activecomponents preclude direct connections because of intermediatecomponents, such as filters, mirrors or prisms, which route ormultiplex/de-multiplex the optical signal channels. In a typical devicea single mode optical fiber is connected to the device at a first, orinput, port and one or more additional single mode optical fibers areconnected at additional ports. Light exiting the optical fiber at aninput port is collimated into a substantially parallel beam by a lens.Additional lenses located at the output ports converge the collimatedbeam into the outgoing optical fiber connected thereto. Lateral andangular offsets of the collimating elements contribute to the signalloss. Since the collimated beam diameter is many times the diameter ofthe fiber core, typically 10 microns, the signal loss due to lateraloffset is reduced. However, the sensitivity of signal loss to angularoffset increases with beam diameter.

[0006] However, the typical macroscopic collimated lenses presentlimitations in miniaturizing devices or increasing the interconnectiondensity without increasing the device or package size considerably.While several methods have been suggested for fabricating a lens on theend of a single mode optical fiber they are not suitable when there mustbe very low signal loss or a miniature device, such as opticalcross-connect switches or multiplex/de-multiplex device.

[0007] Several patents describe how a refractive surface of micro-lensescan be formed or attached to the surface of a single mode fiber. In U.S.Pat. No. 4,268,112 to Paterson a Luneberg type lens with a gradient ofrefractive index is attached to the end of an optical fiber, however thelens diameter is larger than the fiber diameter. In U.S. Pat. No.4,205,901 to Ramsey et al. a single mode fiber is terminated with a coreend region having a core with a graded composition and increasingthickness towards the end of the fiber. In U.S. Pat. No. 4,456,330 toBludau a homogeneous glass rod is welded to the end of a fiber androunded by heat treatment to form a hemispherical lens. However, thesedesign either have significant disadvantages with respect to achieving ahigh interconnection density devices, for example the formation of anadequate lens either increases the diameter of the single mode fiber, ordistorts the edge, thus making the subsequent alignment necessary toachieve low insertion loss difficult, or have a high return loss. Theadditional components increase the complexity of assembly and result inadditional signal loss from splice misalignment.

[0008] Another approach to forming a single mode fiber with amicro-lenses function is to fuse a short section of multimode opticalfiber to the terminal end of a single mode fiber wherein the multimodefiber acts as a gradient index lens, such as in U.S. Pat. No. 4,701,011by Emkey et al. Alternatively the refractive index may be taperedlinearly, such as in U.S. Pat. No. 4,737,004 to Amitay et al., or5,337,380 to Darbon et al. However, it has been found that such devicesare not suitable in miniature devices because they cannot easily bealigned, due to irregularities in the surface shape at the fusion joint,and/or do not shape the exiting beam in a manner compatible with bothlow loss and a high-density of interconnection.

[0009] In U.S. Pat. No. 6,014,483 Thual et al. teach that it is possibleto increase the working of distance of—coupler taught by Emkey et al. byadding a silica spacer between the single mode fiber and the multimode.U.S. Pat. No. 5,457,759 to Kalonji et al. discloses combining insuccession: a piece of graded index multimode fiber, a piece of stepindex multimode fiber and a micro-lens, wherein the terminatingmicro-lens is a curved refracting surface. However, such configurationsappear too difficult to manufacture without increasing or distorting theouter diameter, which is problematic in alignment and assembly.Furthermore, such combinations suffer undesirable back reflection orreturn loss.

[0010] Accordingly, it is an object of the current invention to providea compact optical fiber coupler suitable for the miniaturization ofhigh-density interconnection devices.

SUMMARY OF INVENTION

[0011]FIG. 1 illustrates the benefits of the inventive optical couplerin forming a high interconnection density device 10. Optical signalsarriving from waveguide 1 are transmitted to waveguide 2 afterreflection of surface 13 in device 10. To avoid signal losses theoptical power arriving from waveguide 1 must be efficiently coupledbetween device input port 11 and device output port 12. Light emitted bythe waveguide 1 must be fully collected at output port 12 forre-transmission via waveguide 2 after reflection at surface 13. Theinventive optical coupler modifies the free space propagation of lightemitted by waveguide 1 and the collection of such light into waveguide 2such that ports 11 and 12 may be considered object and image pointsseparated by a working distance (WD) equal to the length, L, of segments14 and segment 15, i.e. WD equals 2L .

[0012] If waveguides 1 and 2 are single mode optical fibers separated byangle alpha the ultimate limitation on decrease the optical device sizeis the optical fiber diameter as well as decreasing alpha forward zero,that is all optical fiber are parallel or nearly parallel and adjacenteach other. To reduce alpha to a few degrees and still utilize only asingle reflective surface as simplest beam path requires a longerworking distance if signal loss is to be avoided as the couplingefficiency is optimum when the optical couplers are positioned at theoptical working distance.

[0013] As alpha approaches 0 the angle of incidence with respect toreflective surface 13 ( alpha/2) results in a desirable reduction inpolarization dependent loss. If reflective surface 13 is an interferencefilter, the long working distance provides the additional benefit ofreducing the angle of incidence, thus minimizing the potentialpolarization splitting, spectral shift characteristic of interferencefilters among several other signal degrading effects.

[0014]FIG. 2 illustrates a first embodiment of the inventive compactoptical fiber coupler wherein efficient coupling is achieved at a longworking distance without increasing the diameter of the waveguide oroptical fiber 21. Optical signals are transmitted through optical fiber21 toward the input port 20 b of device 10 coincident with the terminalend 20 a of compact optical coupler 20. Optical coupler 20 comprises asection of gradient index fiber 22 in optical communication with theterminal end 21 a of optical fiber 21 such that a free space propagatingoptical beam 23 is reduced to a small spot 25 of diameter d at adistance L from the terminal end 22 a of gradient index fiber 22.Placing a reflecting surface or other optical element coincident withthe location of small spot 25 permits selective filtration or routing ofthe optical signals arriving from optical fiber 21 to any other opticalfiber terminated with a corresponding optical coupler at one or moreoutput ports (not shown) of device 10.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 illustrates the optical principles and benefits of theinventive optical coupler in forming high interconnection densitydevices.

[0016]FIG. 2 illustrates the first embodiment of the inventive compactoptical fiber coupler.

[0017]FIG. 3 illustrates the preferred refractive index profile for thegradient index fiber in the inventive compact optical fiber coupler.

[0018]FIG. 4 illustrates the method of joining a single mode fiber to agradient index fiber to form the compact optical fiber coupler.

[0019]FIG. 5 illustrates the method of cleaving the gradient index fiberafter attachment to the single mode fiber to obtain a low backreflection, or return loss.

[0020]FIG. 6 illustrates a preferred embodiment of the compact opticalfiber coupler wherein the angle cleaved face of the gradient index fibercomprises an anti-reflection coating.

[0021]FIG. 7 is a cross section of a portion of an optical deviceshowing the optical coupler mounted in a square-shaped groove fabricatedon a silicon wafer.

[0022]FIG. 8 illustrates the steps in fusion bonding optical fibershaving dissimilar glass transition temperatures or viscosity at thefusing temperature so as to avoid deviation from the circular figure ofthe adjacent portions of the optical fibers.

DETAILED DESCRIPTION

[0023] In order to achieve the long working distance, WD, betweenoptical ports the gradient index fiber has a predetermined profile ofrefractive index, which is illustrated in FIG. 3.

[0024]FIG. 3 illustrates the preferred refractive index profile for thecompact optical fiber coupler. The profile corresponds to equation 1:-

n(r)=n ₀[1-g²r²/2]

[0025] wherein g=2.7 /mm and n₀=1.49 at a wavelength of 1.55 microns.

[0026] The gradient fiber is produced by conventional drawing of a dopedfiber preform fabricated with the corresponding Ge/P-SiO2 glasscomposition profile. The total difference in index within the preform,which corresponds to the gradient in the fiber, is less than about0.001. In the fiber core region, represented by the refractive indexgradient, is preferably greater than about 70 to 80 microns. Thisgradient of refractive index and core diameter results in an opticalcoupler having a working distance of about 550 to 600 microns and a spotsize of about 18 microns when the section of gradient index fiber isabout 815 microns long.

[0027] It should be recognized that both the gradient and core region ofthe fiber could be varied from these preferred parameters to eitherincrease the working distance further, or both the total index changeand core diameter can be increased to obtain substantially the sameworking distance. Since the preferred optical coupler does not increasethe diameter of the single mode fiber, which would limit the potentialinterconnection density, the core diameter is preferably no greater thanabout 75 percent of the single mode fiber cladding diameter, which isabout 125 microns.

[0028] The single mode fiber and gradient index fiber can be placed inoptical communication by numerous means, such as optical contacting,adhesive bonding, index matching fluid or gel, or spacing with an airgap or a homogeneous optical material, such as fused silica, an oxide orsilicon and the like. Such an optical spacer may include or consist ofone of more thin film coatings, such as an anti-reflection coating atthe end of the optical fiber at an air gap spacing. However, a preferredembodiment is fusion bonding the interface between the single mode fiberand the gradient index fiber. A longer than required section of gradientindex fiber is first fusion bonded to the single mode fiber, after whichthe gradient index fiber is shortened to its final length. Methods ofshortening the gradient index fiber include cleaving and polishing.

[0029] In order to achieve the long working distance with the optimumgradient index fiber the length of the gradient index fiber section ispreferably controlled to within an absolute precision of +/− 15 microns,which over a length of about 700 micron represents a deviation about2.5% percent.

[0030] Although the preferred means of forming a planar surface is aconventional cleaving process, this is not entirely compatible withusing a fusion bonding process. It appears that the conventional fusionprocess adversely changes the fracture mode of the gradient index fiberwithin the region where the gradient index fiber should be terminated toobtain the desired long working distance and spot size characteristicssuch that a non-planar surface is formed leading to undesirable backreflection and or signal loss. Not wishing to be bound by theory webelieve the stress state modifies the fracture mode during cleaving fromthe ideal linear propagation necessary to form the perfect planarinterface necessary for low coupling loss, having discovered that asubsequent reduction of the local stress state enables the achievementof low coupling losses with conventional angle cleaving.

[0031] Although a range of heating methods, such as laser, flameannealing, or oven annealing will produce the necessary stressreduction, the simplest approach has been to utilize the low power arcmode provided as a standard setting on the fusion splicing equipment.Alternatively, the entire assembly could be annealed for a functionalequivalent soak time at some temperature below the glass transitiontemperature and softening point of the glass.

[0032] Since final angle cleaving of the gradient index fiber section isdone in the fusion bonding apparatus it is preferable to anneal thegradient index fiber within the fusion bonding apparatus by programmingthe heating cycle and fiber transport accordingly, depending on theheating mode and area of the fusion bonding system.

[0033] This preferred method of stress reduction is illustrated in FIG.4, as fiber 42, which is to be cleaved at dotted line 42 b is annealedwithin the fusion bonding apparatus by localized annealing at region 43about 500 microns distal from the fusion joint 44. Preferably the arcpower is reduced to about 35% of the fusion power while the arc durationis reduced to about 45% of the arc time. The lower power arc isrepeated, typically 4 to 5 times, prior to cleaving the fiber 42 atlocation 42 b.

[0034] An additional aspect of the invention is a reproducible method offabricating the optical coupler with the appropriate length of gradientindex fiber section to achieve the desired small spot size for compactdevices. Accordingly, a preferred method of reproducibly controlling thelength of the gradient index fiber is illustrated in FIG. 5a and 5 b. Inorder to reproducibly manufacture the optical coupler by this method afiber reference 53, such as a removable clamp, is attached to the singlemode optical fiber 51 before a first cleaving step. Surface 53 a offiber reference 53 provides a first fiducial reference a surfaceprotuberance from the optical fiber. Surface 53 a is placed in contactwith a first fixed reference plane 55, which is extended as a dashedline between FIGS. 5a and 5 b ( forming a second fiducial reference).The first fixed reference plane 55 is fixed location on a conventionalfusion-bonding instrument having an integrated fiber-cleaving tool, thecleaving location illustrated by dashed line 59. Optical fiber 51 isthen cleaved to form a clean perpendicular face 51 a. In the preferredembodiment a substantial length of the gradient index fiber 52 is thenfusion bonded to cleaved face 51 a of single mode fiber 51 withoutremoving single mode fiber 51 from the first fiducial reference. Priorto making the final cleave that terminates gradient index fiber 52, theresulting fused single mode fiber and gradient index fiber combinationare remounted by positioning surface 53 a of the fiber reference incontact with a second first fixed reference plane 56 on thefusion-bonding instrument, which serves as a third fiducial reference.Accordingly, the desired cleavage point 52 b has translated and isstabilized in a final location on the fusion bonding/cleaving toolapparatus fixing the final length of the gradient index fiber segmentsuitable for the desired micro-lens function. The length of the gradientindex fiber segment is equal to the distance from the first fixedreference plane 55 to the second fixed reference plane 56. The secondfixed reference plane 56 is easily defined or modified by inserting aspacer block 57 between surface 53 a of the fiber reference and thefirst fixed reference plane 55. The thickness of this spacer blocklength thus determines the length of gradient index fiber segment 52.

[0035] In order to reduce the reflection and insertion losses theterminal end of the optical coupler is a planar surface which deviateslightly from applying perpendicular to the optical axis of the singlemode optical fiber, preferably about 3 degrees. The insertion loss ofthe device can be further reduced by coating this planar surface with anantireflection coating, as illustrated in FIG. 6. Anti-reflectioncoating 63 is deposited on cleaved or polished face 62 b of gradientindex fiber 62. The combination of an angle cleave at face 62 b andanti-reflection coating 63 increases the return loss to a value greaterthan 55 dB.

[0036] The inventive coupler is preferably used in a compact opticalswitch or crossconnect that is fabricated from a monolithic substrate,such as silicon, wherein the photolithography methods can be used tofabricate optical components, preferably translatable mirrors, and theassociated actuator devices. The small spot size of the inventiveoptical coupler allows fixed or translatable mirrors to be reduced insize accordingly.

[0037] An additional aspect of the invention is a method of fabricatingthe inventive optical coupler so that is capable of being mountedprecisely in the final optical device. As the spot diameter ispreferably less than 30 microns, the optical coupler must be fabricatedin a manner that does not interfere with mounting within a tolerance ofseveral microns in order to avoid signal losses. FIG. 7 illustrates thepreferred method of mounting optical coupler 71 in optical device 70 viacross-section transverse to the optical beam propagation direction.Optical coupler 71 is contained within a square-shaped groove fabricatedon a silicon wafer 72. Such grooves are routinely formed by aphotolithographic methods. In order to achieve accurate placement withrespect to the other optical components and ports within optical device70 the fusion joint region must not increase the optical couplerdiameter at the fusion bond, or any region which is to be placed withthe square-shaped-groove 73. Thus, the deviation from the circularfigure of the optical fiber should be less than 5 microns, preferablyless than about 1 micron.

[0038] Avoiding such deviations at the fusion bond requires anoptimization of the fusion process according to the glass transitiontemperatures and viscosity of the glasses of both the single mode fiberand the gradient index fiber, which will vary in the radial directiondue to the composition gradient. Commercially available fusionsplicing/bonding equipment can be utilized to achieve such smooth fusionjoints provided the heating and mechanical movement of the fibers areindependently programmable for incremental adjustments so as toaccommodate a wide range of glass compositions. As FIGS. 8a-e illustratewe have determined that the principal parameters are the arc power,dwell time and fiber pushed together distance and the pull apartdistance (during the arc.) FIG. 8a illustrates a single mode fiber 81and gradient index fiber 82 brought into close proximity immediatelybefore fusion bonding. FIG. 8b illustrates the distortion at theterminal ends of optical fibers 81 a and 82 b from heating. The gradientindex fiber 82 has a lower glass transition temperature or meltviscosity which results in greater rounding at terminal end 82 a afterheating to the same or similar temperature as single mode fiber 81. FIG.8c illustrates the result of the fusing the heated fiber ends by pushingthe ends of fibers 81 and 82 together in that a bulbous protrusion 83form in gradient index fiber proximal to the fusion joint due to theconsiderably lower melt viscosity of the glass. This protrusion 83 canbe removed to form a substantially smooth fusion joint 85, shownschematically in FIG. 8e, by pulling the fibers apart immediately afterfusing but before the molten glass has cooled. The pull apart distanceor stroke will generally be less than the push distance or stroke,depending on the glass compositions, the area heated and the localtemperature. As illustrated in FIG. 8d, excessive pulling produces ataper 84 at the fusion interface, thus optimum conditions can be foundby producing a series of samples by increasing the pull apart stroke afixed increment until the bulge is either eliminated or a taper isproduced. By further incremental adjustment of the aforementionedparameters the deviation from the circular figure of the optical fibercan be reduced to less than 5 microns, preferably less than 1 micron.

[0039] While the invention has been described in connection with apreferred embodiment, it is not intended to limit the scope of theinvention to the particular form set forth, but on the contrary, it isintended to cover such alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

1. A fiber-optic optical coupling assembly comprising: a) a firstoptical waveguide having a first terminal end, b) a section of gradedindex fiber, wherein the first terminal end of said graded index fiberis in optical communication with the first terminal end of the firstoptical waveguide whereby an optical beam propagating from the firstterminal end of the first optical waveguide and exiting the secondterminal end of the graded index fiber is reduced to a diameter d atdistance from the terminal end of the graded index fiber L, wherein d isless than about 30 microns and L is greater than about 220 microns.
 2. Afiber-optic coupling assembly according to claim 1 further comprising anoptical spacer selected from the group consisting of; a. an air gap, anoxide of silicon, index matching fluid and an index matching gel,wherein the optical spacer is between the first terminal end of saidoptical waveguide and the first terminal end of said graded index fiber,whereby the optical beam is expanding from the core section of thesingle mode optical fiber prior to entering said gradient index fibersection.
 3. A fiber-optic coupling assembly according to claim 2 whereinthe optical spacer comprises a thin film coating.
 4. A fiber-opticoptical coupling assembly comprising: a) a first optical waveguidehaving a first terminal end, b) a section of graded index fiber havingindex of refraction gradient characterized by a change in refractiveindex of less than about 0.009 over a core diameter of about 80 micronswherein the first terminal end of said graded index fiber is in opticalcommunication with the first terminal end of the first optical waveguidewhereby an optical beam propagating from the first terminal end of thefirst optical waveguide and exits the second terminal end of the gradedindex fiber.
 5. An fiber-optic optical coupling assembly according toclaim 4, wherein the second terminal end of the graded index fiber isformed by cleaving at an angle of about 3 degrees from a reference planeperpendicular to the optical fibers axis.
 6. An fiber-optic opticalcoupling assembly according to claim 4, the assembly further comprisingan anti-reflection coating at the second terminal end of the gradientindex fiber.
 7. A process for coupling a segment of optical fiber toanother optical fiber, the process comprising: a) cleaving a firstoptical fiber, b) cleaving a second optical fiber, c) fusion splicingsaid first optical fiber to said second optical fiber, d) reducingstress in a section of either the first or second optical fiber proximalto the fusion splice, e) cleaving the stress reduced optical fiber.
 8. Aprocess for coupling a segment of optical fiber to another optical fiberaccording to claim 7 wherein the stress reduced fiber is cleaved about700 microns distal from the fusion splice between the first and secondoptical fibers.
 9. A process for attaching a segment of optical fiber toanother optical fiber according to claim 7, the process furthercomprising the steps of: a) associating a 1st fiducial reference surfacewith said first optical fiber before cleaving said first optical fiber,b) aligning said 1st fiducial reference surface with a 2nd fiducialreference surface associated with the fusion splicing and cleaving locibefore fusion splicing, c) displacing said 1st fiducial referencesurface with respect to said 2nd fiducial reference surface by adistance equal to the predetermined length of said second optical fiberprior to cleaving the stress reduced optical fiber.
 10. A process forforming an fiber-optic optical coupling assembly characterized by areturn loss value greater than 55 dB, the process comprising; a)cleaving a first optical fiber, b)cleaving a second optical fiber,c)fusion splicing said first optical fiber to said second optical fiber,d)reducing stress in a section of either the first or second opticalfiber proximal to the fusion splice, e) cleaving the stress reducedoptical fiber at an angle of about 3 degrees from a reference planeperpendicular to the optical fibers axis to form a terminal end, f)depositing an anti-reflection coating on the second terminal end of thestress reduced optical fiber.
 11. A process for forming an fiber-opticoptical coupling assembly according to claim 10 wherein the stress inthe first or second optical fiber is reduced by localized heating with alaser, micro-flame or low power electric arc.
 12. A process for formingan fiber-optic optical coupling assembly according to claim 10 whereinthe stress in the first or second optical fiber is reduced by localizedheating by repeated discharging an arc having less than 50% of the powerof the fusion splicing arc.
 13. A process for forming an fiber-opticoptical coupling assembly according to claim 10 wherein the stress inthe first or second optical fiber is reduced by localized heating byrepeated discharging an arc having less than 50% of the arc time of thefusion splicing arc.
 14. A process for forming a fiber-optic opticalcoupling assembly according to claim 10, wherein the stress reducedfiber is cleaved about 700 microns distal from the fusion splice betweenthe first and second optical fibers.
 15. A process for forming an fusedoptical coupling between an optical fiber and a gradient index fibercharacterized by the fusion joint having a deviation from the circularfigure of the adjacent optical fiber of less than 5 microns, the processcomprising; a) cleaving a first optical fiber to form a first terminalend, b) cleaving a gradient index fiber to form a second terminal end,c) bringing the first and second terminal ends in close proximity, d)heating the first and second terminal ends above a glass transitiontemperatures characteristic of the glass compositions of at least one ofthe first optical fiber or the gradient index fiber, e) pressing thefirst and second terminal ends into contact such that the lowerviscosity glass forms a bulbous protrusion at the fusion joint, f)pulling the gradient index fiber away first optical fiber before thefusion joint solidifies such that the bulbous protrusion at the fusionjoint is substantially eliminated.
 16. A process for forming anfiber-optic optical coupling according to claim 15 such that the fusionjoint has a deviation from the circular figure of the optical fiber ofless than 2 microns.
 17. A process for forming an fiber-optic opticalcoupling according to claim 15 such that the fusion joint has adeviation from the circular figure of the optical fiber of less than 1microns.
 18. A process for forming an fiber-optic optical couplingaccording to claim 15 such that the diameter of each optical fiber atthe fusion joint is no more than 2 microns less than the diameter of theadjacent section of the optical fiber.
 19. A process for forming anfiber-optic optical coupling according to claim 15 such that thediameter of each optical fiber at the fusion joint is no more than 1microns less than the diameter of the adjacent section of the opticalfiber.